The present invention relates to a system for measuring the carrier lifetime in semi-conductor wafers.
The minority carrier lifetime is one of the most important measures of the electronic quality of crystalline silicon for solar cells. One application of lifetime measurements is in solar cell manufacturing, where there is a need to monitor the quality of the starting wafers prior to solar cell fabrication. In this application a fast and reliable technique for measuring the lifetime in unprocessed wafers is required. Also from a materials research viewpoint, lifetime measurements are a straight forward way to evaluate the effectiveness of novel material refining processes.
There is a substantial amount of prior art on lifetime measurements in crystalline silicon, with discussions of numerous techniques, see e.g., "Lifetime Factors in Silicon" ASTM STP 712, American Society for Testing and Materials (1980). This prior art can be summarized by saying that the best methods generally require finished devices and that for crystalline silicon solar cells the preferred technique is probably the spectral response of the collection efficiency, see e.g., A. G. Milnes and D. L. Feucht, "Heterojunctions and Metal Semiconductor Junctions", Academic Press (1972). This technique measures a quantity related to the lifetime, namely the minority carrier diffusion length. Although the diffusion length may be more directly relevant to solar cell performance, the lifetime itself which can be more directly related to the properties of the material can also be determined in the device from the open circuit voltage decay or short circuit current decay. In many situations, such as screening wafers prior to processing, or monitoring the effect of processing steps on the lifetime, measurements on devices are not feasible. In principle, the spectral response technique can be applied to unprocessed wafers (where it is called "surface photovoltage"), however, in practice this technique is difficult and unsuited to repetitive measurements on many wafers.
In the present invention, a new contactless system for making lifetime measurements has been developed that is a convenient and reliable way to determine the lifetime in unprocessed silicon wafers.